1. Field of the Invention
The present invention pertains to functionalized, discrete silsesquioxanes which are useful, inter alia, for preparing organic and inorganic nanocomposites, to methods of their preparation and their use. Nanocomposites prepared from the functionalized silsesquioxanes have unusual physicochemical characteristics.
2. Background Art
Macroscopic composites of many types are known. Familiar examples include carbon fiber and glass fiber reinforced thermoplastic and thermoset composites. Composite properties generally exceed those predicted from application of the “rule-of-mixtures” based on the properties of the individual components. This increase in physicochemical properties is believed due to interfacial interactions between the dispersed and continuous phases. As the size of the various phases diminishes, the surface area increases, thus increasing the interfacial interactions. At the interfaces of divergent materials, a separate phase, the “interphase” can be hypothesized.
Nanocomposites are composites where the interphase characteristics dominate composite properties due to the very small size of the materials used to prepare the composites. The particles of nanocomposites may be viewed as having a maximum dimension of about 100 nm or less. While substances such as colloidal silica can be supplied in nanometer sizes, the functionality of such silica is limited, and the size and geometry of the individual particles are irregular. Thus, it is impossible to prepare nanocomposites having well defined nanostructural units from such products.
Cage-like silica compounds are known to exist. However, the functionality of such structures is generally limited to hydroxyl (silanol) functionality, if functionality is present at all, thus reducing their utility as nanocomposite building blocks.
Silsesquioxanes functionalized with —OSi(CH3)2H groups are known, as are poly(glycidyl) and similar derivatives prepared by hydrosilylating compounds such as allylglycidylether with the aforementioned Si—H functional silsesquioxanes. However, there is a need to provide additional reactive silsesquioxanes, particularly silsesquioxanes which exhibit high thermal stability and a wide variety of functionalization.
Octaphenylsilsesquioxanes (“OPS”) have been known for some time, J. F. Brown, Jr., et al., J. AM. CHEM. SOC. 86 1120-1125 (1964), and are commercially available. OPS may be produced, for example, by the hydrolysis of phenylsilanes such as phenyltrichlorosilane and phenyltrimethoxysilane. However, the phenyl group itself is considered non-functional, and prior attempts to functionalize it have not proven successful. See, e.g. Voronkov, M. G., et al. “Polyhedral Oligosilsesquioxanes and Their Homo Derivatives,” TOP. CURR. CHEM., 102, 199 (1982); and K. Olsson, et al., ARKIV. KEMI. 17 529-40 (1961). Olsson et al. were apparently successful in octanitrating octaphenyl silsesquioxane to form octakis(p-nitrophenyl)silsesquioxane in quantitative yield. However, the nitro group is an essentially unreactive group relative to use as a bonding group, and Olsson's attempts to reduce the octanitro compound to a useful and reactive octaamino compound were not successful, the authors describing the nitrophenyl-substituted compound as “inert.”
It would be desirable to provide well defined, oligomeric silsesquioxanes which are functionalized with reactive groups which can be employed in controlled chemical bonding. If such functionalized silsesquioxanes were available, a variety of nanocomposites having well defined structure could be created. The resulting products could have numerous uses in fields as wide ranging as improved strength construction materials and photonic crystals or as traps for quantum dots, catalyst particles, etc., to name but a few.